Ammeters. Unnecessary and a fire hazard, or worthwhile?

[Ian McKay]

Hello troops.

I scored a good deal on eBay a little while ago....a new and unused centre console for my 1972 B.

Already mounted in this console were three very nice looking Smith's instruments, one of which was an ammeter.

Upon closer examination, alarm bells started ringing in my head when I noted the huge size of the ammeter's electrical terminals.

I learned from subsequent further reading that the entire output from the alternator has to pass through the instrument en-route to the car battery....hence the very robust electrical connections.

The idea of bringing all that potential energy through the engine firewall into the dash has put me off somewhat.

It seems to me that at best, if the instrument fails, I end up with a non-charging, and eventually flat battery, and at worst, I could end up with a thermonuclear melt down and fire....Am I being overly paranoid here, or are these instruments really useful?

Thanks.....

[Vic Butler]

Evening Ian.
No you are not being paranoid. Ammeters with alternators are not is good idea. As you say the output from the alternator passes through the ammeter and is a fire risk with the current involved. They are fine with dynamos. If you were to go ahead the ammeter must be at least a 60-60 amp version and I believe ones for alternators are different internally.
My recommendation is to fit a voltmeter ensuring that its supply is cut off when the ignition is switched off otherwise it will drain the battery. The wiring for a voltmeter is only light about the same as a sidelight wire.
. Vic Butler

[Peter Cresswell]

Hi Ian
Ammeters were popular on cars with dynamos as they show whether charge from the dynamo is going into the battery (it reads +ve) or the electrical load is drawing current from the battery (it reads -ve). The idea being that this gauge shows that if you want the car to start next morning you keep the electrical demand to a minimum. Ignition and lights are obviously vital systems but you could sacrifice the heater and radio if necessary! This was especially important in the winter and at night. Also the dynamo didn't provide enough charge to carry the load demanded at tick over, and then the car was running solely on the battery. So on the daily commute the battery would slowly run down until the car wouldn't start.
Then three things changed. 1), Alternators were introduced which charged at tick over. 2) the electrical demand increased due to better heaters, electric heated rear windows, electric windows, more efficient heaters, electric fans and so on. 3) Battery technology evolved, but then they started to suddenly fail.
Then to give the driver information that was useful in the 'alternator world', the ammeter was replaced by the voltmeter, as this more accurately reflected the 'state' of the battery. It pains me to say, but Ford led the way on this! When fully charged it should show around 13.5 volts, but below around 11volts the battery is unlikely to start the car on a cold morning.
As your 'B' has an alternator (it is a 45 amp 18ACR type as opposed to early MGBs that had a 21 amp dynamo) and probably by now a modern battery (which will fail suddenly!) a voltmeter is a better instrument to have. So I suggest you swap your ammeter for a Smiths voltmeter. A big advantage, that should allay your concerns, is the voltmeter only needs small gauge wiring across between the terminals of the battery and the voltmeter as the instrument only senses voltage rather than measuring current - which could be as high as 500 - 800amps if a short circuit occurred in an ammeter circuit and would surely result in a loom fire.
The bottom line is keep the other 2 dials and change the ammeter for a voltmeter. It is much better at giving you useful information. You already have a device that measures current going into (or out of) the battery. It is the little red light at the bottom of the tachometer!

[Ian McKay]

Thanks for the replies gents.

It's very impressive just how much knowledge and experience is out there for the asking.

What you have both said backs up my gut feeling regarding ammeters. I think I will pull it out of the console, and install a two inch Smiths time clock in the hole....I find them rather an attractive piece, and I am sure they are a lot safer, more useful, and will not be a potential source of trouble.

There is already a volt meter in the console, so that, coupled with the red warning light on the RPM gauge should tell me all I need to know about the car's electrical system....

Thanks again.